Electrical energy storage device utilizing current collector having anisotropic electricl properties

ABSTRACT

An electrical energy storage device is provided which has a pair of spaced aluminum-lithium alloy anodes with an active carbon cathode interposed therebetween and the anodes and cathodes are separated by an inert spacer material containing a fused salt electrolyte. This device is assembled by enclosing the cathode in a fiber mat, which functions as the spacer, positioning the cathode in a screen basket, closing the basket to contain the cathode, inserting the screen basket into a container, positioning an anode within the container on each side of the screen basket, and hermetically sealing said container.

United States Patent Hacha [451 Feb. 29, 1972v [54] ELECTRICAL ENERGY STORAGE DEVICE UTILIZING CURRENT COLLECTOR HAVING ANISOTROPIC ELECTRICL PROPERTIES.

[72] Inventor: Thomas H. Hacha, Willoughby, Ohio [73] Assignee: The Standard Oil Company, Clevelan Ohio [22] Filed: Mar. 20, 1969 211 App]. No.: 808,877

[52] US. Cl. ..l36/6, 136/83 R [51] Int. Cl. ..H0lm 35/00 [58] Field ofSearch ..136/83, 22, 6,146,145,153,

[56] References Cited UNlTED STATES PATENTS 3,023,262 2/1962 Emmerling et a1 136/120 3,323,869 6/ 1 967 Olstowski ...23/209.1 3,389,964 6/1968 Olstowski ..23/209.1

Primary Examiner-Anthony Skapars Attorney-John E. Jones [57] ABSTRACT An electrical energy storage device is provided which has a pair of spaced aluminum-lithium alloy anodes with an active carbon cathode interposed therebetween and the anodes and cathodes are separated by an inert spacer material containing a fused salt electrolyte.

This device is assembled by enclosing the cathode in a fiber mat, which functions as the spacer, positioning the cathode in a screen basket, closing the basket to contain the cathode, inserting the screen basket into a container, positioning an anode within the container on each side of the screen basket, and hennetically sealing said container.

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FIG-8 I 1 I I I 1 I INVENTOR. THOMAS H. HACHA ATTORNEY ELECTRICAL ENERGY STORAGE DEVICE UTILIZING CURRENT COLLECTOR HAVING ANISOTROPIC ELECTRICL PROPERTIES This invention relates to an electrical energy storage device comprising a cathode separated from adjoining anodes by means of a separator containing within its structure a fused salt electrolyte. More particularly, this invention relates to an electrical energy storage device wherein an active carbon cathode is enclosed in a fiber mat and the entire assembly is placed in a screen basket with a lithium alloy anode positioned on either side and adjacent to the cathode assembly.

The invention is illustrated by reference to the accompanying drawings wherein:

FIG. 1 shows a plan view of an assembled electrical energy storage cell;

FIG. 2 is a cross-sectional view of FIG. 1 taken along plane 22;

FIG. 3 shows one embodiment of the cathode;

FIG. 4 is a cross-sectional view of the cathode along plane 4-4;

FIG. 5 shows a current collector rod;

FIG. 6 is an enlarged fragmentary view of the connector rod and its relationship to the cathode;

FIG. 7 is a cross-sectional viewof FIG. 6 alongplane 77;

FIG. 8 is a further enlarged view, partially in section, illustrating a flexible connection between the cathode and the current collector rod; and

FIG. 9 is a perspective view of an open screen basket.

The attendant advantages of this invention will become more apparent and better understood on referring to the following detailed description covering the assembly of the appar'atus, in connection with the accompanying drawings.

In FIG. 1, braces 58 are spot-welded to prevent collapsing of the container walls 56 in the free space on headroom 60. Braces 58 are made by shaping a rectangular figure out of a strip of metal, such as stainless steel. Lid 62 is positioned on the container and welded thereto,.with current collector rod 24 being disposed within collar 66. A hermetic seal is provided between metallic sleeve 30 and collar 66, as by means of a weld. Fill tube 64 is secured to lid 62 for the purpose of providing access to the interior of the cell for addition of electrolyte, as the need may arise. The anodes 10 in FIG. 1 are arranged in groups of four.

In FIG. 2, the cathode 12 is surrounded by separator 44. The anodes 10 are positioned adjacent to the separator and separated by a screen basket 46.

In FIG. 3, a carbon cathode composite is formed by providing screen 20 and an elongated strip 22, of a highly conducting material, between a pair of graphite current collectors 18 which are then cemented between a pair of carbon plates, 14 and 16.

In a different embodiment shown in FIG. 4, a pair of graphite current collectors l8, constructed from expanded graphite sheet prepared according to the process described in U.S. Pat. No. 3,404,061, are cemented between a pair of carbon plates 14 and 16 and the composite is subjected to heat and pressure to cure and to carbonize the cement.

In FIG. 5, metallic current collector 24 is preferably a rod constructed of tungsten. It has a conical tip 26 at one end and a gas and liquid impermeable seal 28 tightly enveloping the rod adjacent to the conical tip 26. The seal 28 may consist of a metallic sleeve 30 and an inert refractory material 32. The purpose of the seal is to prevent the escape of gas and liquid from the interior of the cell. U.S. Pat. application Ser. No. 618,372, filed on Feb. 24, I967, discloses a type of seal which can be utilized in the cell described here. Said patent application is incorporated herein by reference.

The attachment of current collector rod 24 to cathode 12 is illustrated in FIGS. 6 and 7. Attachment adapter 34 is provided with bore 36 for receiving current collector 24. Notch 38 is cut in adapter 34 to accommodate graphite current collectors 18, screen 20 and strip 22, cementedbetween the graphite current collectors. Notch 38 is in communication with 0 complementary graphite section 40. This section is attached to the adapter and the graphite current collector subassembly by means of cement and pins 42, which are preferably made of graphite. Cement is used to maintain the pins in place.

To effect an electrical connection between the current collector rod 24 and the cathode, a portion of graphite sheets 18 is removed to expose strip 22, as shown in FIG. 8. The current collector rod 24 is inserted into bore 36 of adapter 34 as far as it will go, resulting in displacement of strip 22 by conical tip 26. The contact between current collector rod 24 and strip 22 provides an electrical path from the cathode and through the collector rod to a point externally of the cell. Also, a parallel electrical connection is efi'ected between the cathode and collector rod via the press fit between the collector rod and the graphite adapter.

Cathode 12 is wrapped in separator 44 and then placed into a screen basket 46, as shown in FIG. 9. The basket consists of a thin metal strip 48 which is welded to screens 52 and 54. When the wrapped cathode is positioned in basket 46, screen 54 is raised against metal strip 48.

Screens 52 and 54 are preferably stainless steel and may be from 20- to over lOO-mesh size with 15-35 percent open area. The screens serve the purpose of inhibiting growth of dendrites which emanate from the aluminum-lithium anodes. Provisions of screens to prevent dendritic growth is described in U.S. Pat. application Ser. No. 518,113, filed on Jan. 3, 1966, now U.S. Pat. No. 3,428,493. This application is incorporated herein by reference. Other means for constricting the growth of dendrites such as cloth, fibers, etc., are also effective.

The anodes of this assembly, which may be arranged in groups of four, comprise lithium or a lithium alloy, such as aluminum-lithium, indium-lithium, tin-lithium, lead-lithium, silver-lithium, copper-lithium, etc. Ternary lithium alloys can likewise be used.

The preferred anodes are the highly reversible aluminumlithium electrodes. These electrodes can be produced by combining lithium with aluminum and thus producing a preformed alloy of aluminum and lithium, or, electrochemically, by charging a substantially pure aluminum bar in an electrolyte containing lithium ions to the extent of about I ampere hour per gram of aluminum, whereby lithium is diffused into the aluminum bar.

The solid aluminum-lithium alloy anode comprises aluminum and incidental impurities in amounts of from about 70-95 weight percent based on total composition, and from about 5-30 weight percent lithium. The lithium composition is critical. At 5 percent of lithium and below, the capacity is inadequate for practical purposes while above 30 percent lithium, the discharge curve is not flat but is a gradient downward. Incidental impurities such as, for example, copper, magnesium, manganese, indium, and iron may be present in amounts less than 10 weight percent. An anode consisting entirely of lithium can also be utilized, however, because of its low melting point, i.e., 186 C., it will be liquid at the operating temperature and its discharge profile will be a gradient downward.

The aluminum-lithium electrode is capable of storing lithium metal from the electrolyte without forming an extensive liquid. Hence, at an operating temperature below its melting point, the electrode remains solid and is capable of difiusing the lithium metal from the electrolyte through its structure. It has been found that on charging the cell comprising the aluminum-lithium electrode, the electrode expands, whereby the lithium metal from electrolyte enters the electrode structure.

On discharge, the lithium metal leaves the electrode structure, resulting in its contraction. As is evident, the electrode structure must be able to withstand the stresses of expansion and contraction and for this reason, the aluminum-lithium electrode is preconditioned prior to use.

The preconditioning takes the form of slow charge and slow discharge of the electrode. This slow preconditioning results in an electrode of substantially uniform aluminum-lithium distribution which facilitates takeup and release of the lithium metal upon subsequent fast charge and fast discharge of the electrode. If the initial charge and discharge preconditioning cycles are carried out too rapidly, local regions of liquid are built up resulting in pitting of the electrode. This pitting of the electrode is deleterious in that it promotes cracking and general deterioration of the electrode. Evidence of pitting is visible in the form of lithium agglomeration. Aluminum-lithium electrode cycled by slow charge and discharge shows a fine uniform distribution of the lithium metal in the aluminum. The aluminum-lithium anode-alkali halide molten salt system is more fully described in US. Pat. application Ser. No. 518,473, filed Jan. 3, 1966, now US. Pat. No. 3,445,288. This application is herein incorporated by reference.

The aluminum-lithium electrode is characterized as a constant potential electrode. This means that when the aluminumlithium electrode is charged to a potential which must of necessity be below the decomposition potential of the particular electrolyte in the system, the discharge should be at a constant voltage until the very end when the system becomes fully depleted. In actual practice, however, it has been found that the aluminum-lithium electrode does not discharge at a constant potential. The potential drop of the electrode is a gradient downwardly. This problem has been solved by removing the surface film from the electrode in an inert atmosphere and maintaining the electrode in an inert atmosphere or submerged in an inert hydrocarbon until it is ready for use. Removal of surface film as described will result in an aluminum-lithium electrode which is truly a constant potential electrode. The procedure for removing the surface film from an aluminum-lithium electrode is more fully described in US. Pat. application Ser. No. 550,239, filed on May 16, I966, now US. Pat. No. 3,501,349, which is hereby incorporated into this disclosure by reference.

Prior to use, the anodes are acid washed in dilute hydrochloric acid to remove oxide films which inhibit activity of the electrode. Removal of the oxide film can be accomplished in other ways, as described in US. Pat. application Ser. No. 550,239, filed May 16, 1966. The anodes are washed with water to remove the acid and then with alcohol to remove water. The clean anodes are subjected to vacuum bake-out to remove any volatile impurities. The bake-out program consists of heating the anodes to about 300 C. over a period of 1 hour while maintaining a vacuum of Torr, holding the anodes at 300 C. for about I hour, then cooling to room temperature for about 1 hour. The anodes are immersed in molten salt and held there for a period of time to further remove impurities. Upon removal from the salt and cooled to room temperature, the anodes are ready for assembly.

Cathode 12 is constructed from activated carbon in the form of a plate. The active carbon utilized in the preparation of the cathode has a surface area in the range of l002,000 m. /g., and preferably in the range of SOD-L500 m. /g., as measured by the B.E.T. method. The surface area is mainly internal and may be generated by numerousactivation methods, some of which are hereinafter discussed. The pores in the activated carbon must be of sufficient size to permit electrolyte penetration.

In general, active carbon contains more than 80 percent carbon, as well as hydrogen, nitrogen, oxygen, sulfur, and inorganic salts that remain as an ash on combustion.

The initial stage in the preparation of an active carbon is carbonization or charring of the raw material, usually in the absence of air below 600 C. Most any carbon-containing substance can be charred. After the source material is charred,

the second step is activation. The method used most extensively to increase the activity of carbonized material is controlled oxidation with suitable oxidizing gasses at elevated temperatures. Most of the present commercial processes involve steam or carbon dioxide activation, between 800 and l,O0O C., or air oxidation between 300 and 600 C. Alternately, gases such as chlorine, sulfur dioxide and phosphorous may also be used. The time required for activation varies from 30 minutes to 24 hours, depending on the oxidizing conditions and the quality of active carbon desired. Inhibitors or accelerators can be mixed with the carbon to develop the increased activity. Other activation methods include activation with metallic chlorides and electrochemical activation. The latter is a process whereby the capacity of an electrode can be increased by electrochemical cycling.

Another general method of activation is the dolomite process. Substances such as dolomite, sulphates and phosphoric acid are mixed with the carbon. On activation, the material continuously releases a uniform distribution of oxidizing gases to the carbon surface.

Some activated carbons are prepared from hard and dense materials. These materials are usually carbonized, crushed .to size, and activated directly to give hard and dense granules of carbon. In other cases, it is advantageous to grind the charcoal, coal, or coke to a powder, form it into briquettes or pellets with a tar or pitch binder, crush to size, calcine to 500-70 0 C., and then activate with steam or flue gas at 850-950 C. The latter procedure gives particles with a tailor-made structure which are easier to activate because they possess more entry channels or macropores for the oxidizing gases to enter and the reaction products to leave the center of the particles.

In the preparation of the carbon electrodes of this invention, either carbon plates or loose granular carbon particles can be used. In the case of plates, a sheet of graphite current collector is cemented between a pair of carbon plates, following which the composite is subjected to heat and pressure to cure and to carbonizethe cement. In a different embodiment, acarbon cathode composite is formed by providing a screen and an elongated stripof a highly conducting material between a pair of graphite current collectors which are then cemented between a pair of carbon plates.

When loose granular carbon particles are used for the cathode material, the cathode is produced by feeding a layer of the carbon particles containing a carbonizable binder into a mold, laying a graphite current collector, or a current collector assembly, such as described above, over the particulate carbon, pouring another layer of particulate carbon with binder over the graphite current collector and heating and compressing the contents of the mold. The binder upon heating and compression flows to produce a plate cathode. The cathode is subsequently heated to carbonize the binder.

US. Pat. application Ser. No. 748,943, filed on July 31, 1968, now abandoned, describes the manufacture of the carbon cathode. This application is incorporated herein by reference for the purpose of rendering this disclosure complete.

Prior to use, the carbon cathode is preconditioned, as for. example, under a vacuum of 28 inches of mercury by charging at about 4 amperes to 3.2 volts and then discharging to 0.7 volt at l6 amperes. On the following cycle, the cathode is charged to 3.3 volts where it is held for about 6 hours and then discharged to 0.7 volt. On the succeeding six to eight cycles, the cathode is cycled between 3.35 volts and 0.7 volt at c/2 rate. The preconditioning process is completed with about 10 additional cycles between 3.35 volts and 0.7 volt at lc rate. A lc rate is defined as a constant current charge or discharge rate that will discharge the cell between prescribed voltage limits in one hour. The preconditioning process need not be limited to the conditions employed above, since many variations thereof are possible.

In addition to preconditioning, the cathode may be chlorinated to increase its capacity and/or reduce cathode conditioning time. Chlorination is carried out prior to the preconditioning process and consists of treating the cathode with chlorine gas first at a low temperature, then at a high-temperature.

The cathode is surrounded by a separator which can be in the form of a fiber mat, felt, plate, etc. The mat can be fabricated from such refractory materials as beryllium oxide (BeO), thorium oxide (ThO magnesium oxide (MgO), lithium aluminate (Li/310 boron nitride (BN), silicon nitride Si N aluminum nitride (MN), and mixtures thereof. Some embodiments of the separator are described in U.S. Pat. application Ser. No. 625,053, filed on Mar. 22, 1967, now U.S. Pat. No. 3,510,359. This application is incorporated herein by reference.

The separator contains electrolyte within its structure. The amount of electrolyte contained within the separator may vary considerably. The separator should contain sufficient electrolyte to provide a continuous path for the ions from one electrode to the opposite electrode. The electrolyte is a medium comprising a source of dissociated metal and halide ions which are mobile and free to move in the medium. Fused salt mixtures containing, for example, sodium chloride, calcium chloride, calcium fluoride, magnesium chloride, lithium chloride, potassium chloride, lithium bromide and potassium bromide can be used. The lower melting point electrolytes are desirable, however, it is contemplated by the present invention that the operating temperature of the cell may reach as high as about 600 C.

Typical examples of materials which can be used as electrolytes include salts of metals. Specific examples of useful binary salt electrolytes are lithium chloride-potassium chloride, lithium bromide-potassium bromide, lithium fluoride-rubidium fluoride, lithium chloride-lithium fluoride, lithium chloride-strontium chloride, calcium chloride-lithium chloride, lithium sulfate-potassium chloride, and mixtures thereof.

Examples of ternary molten salt electrolytes are calcium chloride-lithium chloride-potassium chloride, lithium chloride-potassium chloride-sodium chloride, calcium chloride-lithium chloride-sodium chloride, and lithium bromide-sodium bromide-lithium chloride.

Especially preferred systems, when using an aluminum-lithium negative electrode, are those of potassium chloride-lithium chloride and lithium bromide-potassium bromide and mixtures thereof. A lithium chloride-potassium chloride system of 41 mole percent potassium chloride and 59 mole percent lithium chloride forms a eutectic which melts at 352 C. and has a decomposition voltage of about 3.55 volts.

As a specific embodiment of this invention, the following electrical energy storage device was assembled.

EXAMPLE Container 56 is constructed from 0.010 inches 304 stainless steel and has the outside dimensions of 8.0 6.0 1.0 inches. The container is washed for -20 seconds with a solution containing approximately equal amounts of nitric and hydrochloric acid and is examined for leakage. As with all components, the container is vacuum baked for a duration of several hours at about 500 C.

The carbon cathode shown in FIGS. 2, 4, 7 and 8 contains a tellurium additive of the type more fully described in the copending U.S. Pat. application Ser. No. 808,876, filed Mar. 20, 1969 now U.S. Pat. No. 3,567,516, and is used in its discharged state. Screen is constructed from tungsten and is 30-mesh size. Strip 22 is also composed of tungsten and has dimensions of SygXAXl/ZOO inches. Current collector24 is a tungsten rod one-eighth inch in diameter. Graphite current collectors 18, corresponding dimensionally with the carbon cathode, are constructed from 0.010 inch graphite sheet having anisotropic electrical properties and prepared from expanded graphite particles compressed together by the process described in U.S. Pat. No. 3,404,061. Listed below are some of the properties of the expanded graphite sheet material employed.

Thermal Conductivity at 2,000 F:

(surface plane or a direction) (BTU-ftJsq. ft. hr. F.)

(through thickness or c direction) (BTU-ftJsq. ft. hr. F.) 2 Electrical Resistivity:

Specific Resistivity (surface plane oradirection) (10 ohm/cm.) 8

Area Resistivity (surface plane or a direction for 0.005-in-thick tape) (ohms/square) The outside dimensions of the cathode are 6.0X5.75X0.642 inches and contains about 20 inches of carbon. The cathode is preconditioned before use. I

The cathode is wrapped into a 0.050 inch thick, boron nitride fiber mat which contains about 30 grants of lithium chloride-potassium chloride salt of the composition 59 mole percent lithium chloride 41 mole percent potassium chloride having a melting point of 352 C. The wrapped cathode is placed into screen basket 46 constructed from -mesh stainless steel. The screen basket is held closed when it is assembled in the container.

The screen basket, together with the wrapped cathode, is lowered into the container. A set of four aluminum-lithium anodes, containing 13 percent lithium, is positioned between the screen basket and the wall of the container on one side, and another set of four anodes is positioned in a like manner on the other side. The anodes are in a discharged state. The overall dimensions of the set of four anodes are 527/32X63/ 32X1/20 inches. The size of the screen basket is selected so that the upper edge thereof projects above the cathode. The projecting upper edges of screens 52 and 54 are spot welded to the walls of the container to contain the anodes in place. Strip 48 is bent over to maintain the separator mat and the cathode in place.

Rectangular braces 58 constructed from stainless steel are spot welded, as shown in FIG. 11. Lid 62 is welded to the container with the current collector rod 24 being disposed within collar 66. A hermetic seal is obtained between metallic sleeve 30 and collar 66, by means of a weld. Fill tube 64 is secured to lid 62 for the purpose of providing access into the interior of the cell for adding electrolyte, as the need may arise.

With the lid welded in place, the cell is heated above the melting temperature of the lithium chloride-potassium chloride electrolyte and evacuated to about 100 microns of pressure to displace entrained gases in the separator mat with electrolyte. This step also facilitates penetration of the separator mat with electrolyte. Fused lithium chloride-potassium chloride electrolyte is added to the cell until the level rises in the fill tube 64. The cell is then charged to 3.3 volts. Since the electrodes are in their discharge state, some of the electrolyte is expelled from the carbon cathode on charging. This results in an overflow of the salt from the container. As the final step in this method of assembly, the fill tube is constricted and welded shut.

The above cell has a volume of 44.6 inches and weighs 2.9 pounds. The cell is discharged to 1.0 volt with a constant current discharge rate of 30 amperes giving the following characteristics:

6.7 milliohms 50 ampere hours 98.7 watt hours 34 watt hours per pound 2.21 watt hours per inch Cell Resistance Capacity assembly and flexibility of the component parts. Simplicity of l assembly resides in the fact that it is essential only to slide the electrodes into a container and place a lid thereon. The flexible feature accommodates expansions and contractions of the cell without component cracking or failure. This is possible because of flexibility between the cell components i.e., the flexible connections between the graphite adapter and the carbon cathode and the flexibility of the boron nitride fiber separating the anodes and the cathodes). Thin and flexible walls of the container provide compensation for the change in volume of the free electrolyte during charge and discharge of the cell. The walls of the container may be extra thin since atmospheric pressure will provide support as the cell is operated below atmospheric pressure. The cell configuration provides good anode electrical connection to the cell container by virtue of the anode contact over both sides of the container.

I claim: I E. An electrical energy storage device comprising in combination:

A. a thin, flexible-walled, metallic container; J B. an electrolyte in said container consisting essentially of one or more of the halides of the alkali metals, the alkaline earth metals and combinations thereof; C. a carbon electrode in contact with said electrolyte and consisting essentially of: a. a pair of flat, closely adjacent parallel plates of amorphous, activated carbon, a pair of flexible graphite sheet current collectors consisting of compressed expanded graphite particles, and having anisotropic electrical properties, said current collectors being in laminar contact with the inside surfaces of the parallel carbon plates,

an elongated metal strip consisting of tungsten, disposed between said pair of graphite current collectors,

. a metallic current collector rod, also consisting of tungsten, in immediate contact with the elongated tungsten strip and indirectly in contact with the graphite current collector by means of a graphite adapter cemented to the carbon electrode providing an electrical path from the carbon electrode through the collector rod to a point externally of the cell;

D. a separator in the form of a flexible fiber mat surrounding the carbon electrode, said fiber being a material selected from the group consisting of beryllium oxide, thorium oxide, magnesium oxide, lithium aluminate, boron nitride, silicon nitride and aluminum nitride, and mixtures thereof;

E. at least two aluminum lithium electrodes positioned on opposite sides of the carbon electrode, said lithium aluminum electrodes being in contact with the container wall;

F. a metallic screen basket enclosing said carbon electrode and separator and situated adjacent to the aluminum lithium electrodes, said metallic screen providing means for inhibiting the growth of dendrites on the surface of the aluminum lithium electrodes;

said device being operable above the melting point of the electrolyte. 

